1. Field of the Invention
The present invention relates to ink jet printing devices, and more particularly, to such devices which utilize a plurality of orifices extending in at least one row along a printing liquid supply reservoir; the orifice plate, or a remote parallel pressure plate, being stimulated to produce traveling vibrational waves along its length so as to induce a series of uniform filaments and droplets coming from the orifices.
2. Prior Art
Ink jet printing devices of the type generally referred to above are disclosed, for example, in Houser U.S. Pat. No. 3,701,476 and Mathis U.S. Pat. No. 3,701,988. In such devices generation of the series of uniform filaments and droplets in each of the orifices in the rows of orifices formed in the orifice plate is accomplished by mechanical stimulation where a stimulating probe contacts the upper surface of the orifice plate and causes a series of bending waves to travel lengthwise along the orifice plate in the direction of extension of the rows of orifices.
Due to the fact such traveling waves tend to be reflected by the end supports, such as the manifold forming the fluid reservoir above the orifice plate which engages the upper surface of the orifice plate so as to define the active area thereof, some means must be utilized for damping or preventing the reflection of such traveling waves along the orifice plate so that they do not interfere with the main wave propagation. One means of accomplishing this has been the use of wedged shaped dampers formed of, for example, a polyurethane material as is disclosed in the above referred to patents.
These dampers are generally formed after partial assembly of the orifice plate to the manifold so that they can be poured into place on the surface of the orifice plate and allowed to set in a manner such that they define wedged shaped members extending from adjacent the ends of the rows of orifices upwards towards the ends of the manifold where they engage the orifice plate to define the active area thereof. It has been discovered, however, through careful observation thereof, that the damping devices are not as efficient as desired since they do permit a substantial amount of reflected vibrational waves to travel backward from the end of the orifice plate and interfere with the main wave propagation which in turn results in a reduction in uniformity of both filament and droplet formation.
It has been observed that although the polyurethane damping devices are formed in situ, the leading edge pointing towards the direction of approach of the traveling wave in the orifice plate does not actually taper to zero thickness due to the natural surface tension of the liquid polyurethane when it is being poured. Thus, a vertical wall of somewhat concave lateral cross section exists at the beginning of the damper. This wall tends to cause immediate reflection of the traveling wave thus producing secondary interferring waves traveling back towards the stimulator from the damper.
In addition, the use of polyurethane bumpers produces somewhat of an impedance mismatch between the material of the bumper and the material from which the orifice plate is made. This results in an increase in velocity of the waves generated in the damper due to the fact that they are compression waves rather than transverse flexural waves as is the case in the orifice plate. This is true even though the damping material has a lower density and Youngs modulus than the orifice plate. However, it is possible that with appropriately shaped dampers, the effect of impedance mismatch of the materials can be minimized.